An optical fiber is conventionally constituted of an optical core, which transmits an optical signal, and of an optical cladding, which confines the optical signal within the optical core. To that end the refractive index of the core, nc, is greater than the one of the cladding, ng. An optical fiber is generally characterized by a refractive index profile that associates the refractive index n with the radius r of the optical fiber: the distance r with respect to the center of the optical fiber is shown on x-axis and the difference Δn between the refractive index at radius r and the refractive index of the optical cladding is shown on y-axis.
Nowadays, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber, whereas in a single-mode fiber, the higher order modes are cut-off or highly attenuated.
Multimode fibers are commonly used for short-distance applications requiring a high bandwidth, such as local area networks (LANs) and multi-dwelling units (MDUs), more generally known as in-building networks. The core of a multimode fiber typically has a diameter of 50 μm, or 62.5 μm and an alpha refractive graded-index profile. In such applications, the conventional optical fibers can be subjected to unintended bending, which can cause inherent losses and thus modify the mode power distribution and the bandwidth thereof.
The key parameters that ensure good performances of multimode fibers in multi-gigabit Ethernet communications are the bend-loss resistance and the bandwidth.
A known solution to improve bend-loss resistance of multimode fibers consists in adding a depressed-index portion between the graded-index core and the cladding. This depressed-index portion, usually called a depressed trench, has a negative refractive index difference with respect to the optical fiber cladding, and its position and size are designed so as to avoid degradation of the bandwidth.
However, while the presence of a depressed trench improves the bend-loss resistance of the guided optical modes, it also leads to additional parasitical modes, called “leaky modes”, to co-propagate with the guided optical modes. Optical rays being partially reflecting at the depressed trench interface, the leaky modes exhibit additional losses (commonly called “leakage losses”) compared to the guided optical modes coming from the refractive index profile itself.
The leaky modes are also present within a conventional multimode fiber, i.e. a multimode fiber having no bend-loss resistance means, but they are nearly inexistent in practice, because the level of their leakage losses is extremely high. On the other hand, with a known trench-assisted design of optical fiber, the leakage losses of the leaky modes are so reduced that the leaky modes propagate over several meters and even more, that is critical for compatibility with a conventional multimode fiber. Under OFL (OverFilled Launch) conditions, it was indeed found that the emergence of leaky modes disturbs the characterization measurements, the core size (a) and numerical aperture (NA) measurements being in particular overestimated. This is problematic because the interconnection between fibers requires tight tolerances.
It would be therefore efficient to provide a multimode optical fiber having a limited impact of the leaky modes on the optical characteristics (like the core size and numerical aperture) and still providing a bend-loss resistance, while allowing a broad modal bandwidth.
The patent documents US 2009/0154888, US 2008/0166094, JP 2006/47719, US 2011/0123161, US 2010/0067858, for examples, relate to a graded index optical fiber having a depressed trench within the cladding for reducing bending losses. However, none of these documents discloses a solution to overcome the problem of impact of the leaky modes on the optical characteristics.
The patent document US 2011/058781 also relates to a trench-assisted multimode optical fiber. This document proposes to enhance the bend-loss resistance of the optical fiber by defining the volume of the depressed trench as being comprised between −40 μm and −30 μm. This invention gives a rule that limits the deleterious effect of the leaky modes on the numerical aperture (i.e. on the divergence of light spot output the optical fiber) but does not provide any solution for minimizing the impact on other optical characteristics, like the optical core size for instance. Indeed, the leaky mode leads to an artificial core size enlargement during characterization measurements (the core looks larger than it actually is). Thus, this known solution is not optimal.